Clutches in which two clutch elements can disengage when the torque being transferred exceeds a predetermined value are known, Usually, one of the clutch halves or elements, or parts or components thereof, are shifted axially, or radially, against a spring force, which spring force determines the overload level of the clutch. Clutches of this type are used to protect machines and other devices and plants against overload. The torque being transferred is limited at a suitable position within the drive train of the machine or device, so that it cannot be overloaded and no damage can result. Inter-engaging overload clutches, that is, positive drive clutches which can disengage upon overload have a distinct advantage with respect to friction clutches. The frictional value of friction clutches is variable and difficult to determine. Positive engagement clutches, thus, can be set for comparatively determined and precise overload torque values, without relying on the uncertain frictional value with respect to maximum transmitted torque.
A typical engagement arrangement of the clutch halves utilizes facing teeth, or gear-like elements which, upon disengaging of the clutch, disengage from each other. Disengagement is effected by shifting at least one of the two clutch elements, or, by shifting the teeth or tooth gaps relatively to each other. Other types of positive engagement clutches, for example claw clutches are known and used.
After the clutch is disengaged due to an overload, the clutch usually remains in disengaged position until manually reengaged. Manual reengagement is effected by shifting the movable clutch element to again obtain interengagement of the clutch parts with respect to each other. The shift movement is usually axial.
Upon disengagement of one clutch element from the other, and resulting disengaged position of the clutch elements, the clutch elements can be reengaged precisely provided that the interengaging elements fit against each other, in other words, that teeth formed on one clutch element, for example, fit into tooth gaps of the other one.
Various types of machinery require reengaging of a driven part with a drive part only at predetermined angular positions. There are cases in which reengagement of the clutch should occur only upon a predetermined relative angular relationship of the clutch halves or parts with respect to each other, so that the drive side of the clutch will be engaged with a driven side only at the predetermined angular relationship. This is necessary when it is desired to obtain a predetermined relationship of a machine which, for example, operates in cadence with a production line; or to prevent starting of a machine, upon reengagement of the clutch, at a maximum loading or at an otherwise undesired or even possibly dangerous condition.
It is, of course, possible to manually rotate the clutch elements, and parts coupled thereto of the driven machine with respect to the drive shaft until the desired condition for reengagement of the clutch is obtained. Such manual alignment is difficult and time-consuming. Additionally, errors and misalignment may result since, and depending on the individual condition, may be dangerous for the operator as well as for the machine. This is of particular importance if the torque to be tansferred is substantial, or large masses are to be driven, and, particularly, to be accelerated.